1. Field of the Invention
This invention concerns a magnetoresistive transducer and particularly a magnetoresistive transducer made of thin layers usable to detect magnetic fields.
2. Description of the Preferred Embodiments
In the various fields of application of thin layer magnetoresistive materials (read heads for magnetic recordings, magnetometers, compasses, various types of detectors), all components are now made with thin layers of magnetic alloys such as "Permalloy": Ni.sub.80 Fe.sub.20.
The physical phenomenon used is anisotropic magnetoresistance which results in a resistivity expressed as follows: EQU .rho.=.rho..sub.0 (1+.delta.cos.sup.2 .theta.) (1)
where .theta. is the angle between the magnetization and the current in the material, and .delta. is the relative variation of resistivity between the two extreme situations: magnetization parallel to or perpendicular to the current (typically .delta.=0.01). The term .rho.o is a constant.
These types of thin layers have a number of disadvantages that restrict component performances, for example:
--resistivity is a function of the magnetization orientation, and is related to the value to be measured (the amplitude and/or orientation of an external magnetic field) by a frequently complex behavioral law (non-linear) or even by non-unique values (hysteresis).
--the presence of noise intrinsically related to the discontinuous and non-deterministic nature of the magnetization process of a ferromagnetic: Barkausen noise.
--the fact that the magnetic field orientation reference is relative to the current, which is an implementation constraint.
--temperature highly dependent on the average resistivity and the magnetoresistance .delta. (typically 1000 ppm/.degree. C.) of this type of alloy close to the ambient temperature.
--a low average resistivity of the order of a few tens of .mu..OMEGA..cm implying complex transducer geometries (coils) in order to achieve resistance values that are easy to measure. Moreover, this excludes the construction of detectors in which the current circulates perpendicular to the plane of the thin layer. This possibility may be very interesting from the integration point of view, particularly in the magnetic recording field as described in French Patent request number 90 09301.
Devices are also known using metallic magnetic multi-layers with larger effects where .delta.=0.1 (at ambient temperature and for the same values of the saturation field)--independently of the relative orientation of current magnetization, but without correcting the other weaknesses mentioned. In particular, concerning the variation with temperature and the low value of the average resistivity.
It has also been observed that the conductance of a metal/insulation metal tunnel junction in which two electrodes consist of itinerant ferromagnetics, where itinerant means that electrons carrying magnetic movement participate in electrical conduction (typically 3d magnetic metals and their alloys) depends on the relative orientation of magnetization vectors on each side of the barrier. More precisely, if .theta. is the angle between these two vectors and d is the barrier width, the conductance is: ##EQU1##
where .lambda. is tunnel length that depends on the material, typically between 1 and 100 nanometers, and .DELTA. is a modulation amplitude between 0 and 1. The term G.sub.0 is a constant.
The document T. MIYAZAKI et al. J.M.M.M. 98, L7 (1991) contains a list of values of .DELTA. observed to date, from which a maximum effect .DELTA.=0.14 is deduced at ambient temperature for an Ni.sub.82 Fe.sub.18 /Al.sub.2 O.sub.3 /Co junction.
It is also well established that co-evaporation or co-atomization methods can be used to make thin layers consisting of a dispersion of fine magnetic metallic particles in various insulating matrices (Al.sub.2 O.sub.3, SiO.sub.2, BN . . .). For metal phase fractions by volume of less than 50%, in the case in which metal and insulating phases cannot be mixed, and if these materials are deposited on an unheated substrate, quasi-spherical particles are generally obtained with a diameter of a few tens of Angstroms completely insulated from each other by the insulating matrix that may be amorphous (SiO.sub.2, Al.sub.2 O.sub.3) or crystallized (BN). Under these conditions, and as demonstrated experimentally for the Ni/SiO.sub.2 system in the document written by J. I. GITTLEMAN et al, Phys. Rev. B5 (9), 3609 (1972), the system may simultaneously have a superparamagnetic behavior and electrical conductivity in low field dominated by the tunnel effect between particles.
The superparamagnetism concept refers to the fact that each magnetic particle behaves like a classical magnetic moment of value VM.sub.S (where V is the volume of the particle and M.sub.S is the magnetization of the constituent magnetic material), in which the orientation fluctuates thermally with a characteristic time: ##EQU2##
where .tau..sub.0 is a constant that varies little with the material and is of the order of 10.sup.-10 s, and E.sub.b is an activation energy related to the various magnetic particle anisotropies. The term e.sub.k is a constant. For a particle with uniaxial anisotropy: EQU E.sub.0 =KuV (4)
where Ku is the anisotropic constant and V is the particle volume. Note that proportionality of Eb to the particle volume goes beyond the special case of activation energy associated with anisotropy.